This invention relates to an electronic fuel control method and apparatus for fuel injection engines.
Conventional electronic fuel control systems for fuel injection engines are classified roughly into so-called intake air flow rate sensing systems and speed density systems. The former measures the amount of intake air directly by using a vane-type or hot wire-type air flow sensor. The latter measures the amount of intake air indirectly based on throttle opening or pressure inside the intake pipe.
A typical example of the intake air flow rate sensing system is the L-Jetronic system (referred to as the "L-J system" hereinafter), characterized by a narrow measurable range of flow rates but exhibiting outstanding measurement precision. More specifically, the L-J system is capable of highly accurate measurement of the amount of intake air in an operating region where the amount of intake air is low. A typical example of the speed density system is the D-Jetronic system (referred to as the D-J system hereinafter), characterized by a broad measurable range of flow rates but exhibiting a lower level of measurement precision than the L-J system. The reason for this is that the amount of intake air fluctuates depending upon the operating state of the engine, the outside air conditions and the like, even though the intake pipe pressure and engine rpm can be measured accurately. This fluctuation becomes especially pronounced in the low rpm region.
Thus, the two aforementioned systems have their advantages and disadvantages but measurement precision is sacrificed at some part of the operating region regardless of which system is adopted.
A fuel control apparatus proposed as a solution to the above problem has been disclosed in Japanese Patent Publication No. 59-7017. The disclosed apparatus has an engine intake air flow rate sensor, an intake pipe internal pressure sensor and an engine rotational speed sensor connected to control means for controlling the amount of fuel injection. Control of the amount of fuel injection based on the intake air flow rate sensing system and control of the amount of fuel injection based on the speed density system are switched between in response to the operating region in such a manner that the advantages of both systems manifest themselves. Specifically, a value indicative of the amount of intake air sensed by the intake air flow rate sensor is used in determining the operating region. In an operating region where the value of the amount of intake air is less than a certain value, a fuel injection valve is operated on the basis of output signals from both the intake air flow rate sensor and engine rotational speed sensor in accordance with the intake air flow rate sensing system. In an operating region where the value of the amount of intake air is greater than a certain value, on the other hand, the fuel injection valve is operated on the basis of output signals from both the intake pipe internal pressure sensor and engine rotational speed sensor in accordance with the speed density system without relying upon the sensed amount of intake air.
With a fuel control apparatus of this type, however, the amount of intake air is "measured" indirectly by calculating the pressure inside the intake pipe in accordance with the speed density system in an operating region in which the intake air flow rate is construed to be high, as described above, and the amount of fuel injection is decided on the basis of the amount of this indirectly "measured" amount of intake air. Consequently, an error develops between the actual amount of intake air and the indirectly "measured" amount of intake air. In other words, an error develops between the actual air-fuel ratio and target air-fuel ratio in the operating region where the intake air flow rate is high. This results in poor fuel efficiency or a deterioration in the exhaust gas characteristic.